Effective implementation of laser microfabrication techniques in transparent media at depth up to 4 mm by high NA focusing ultra-short pulse laser beams dictates the need to compensate spherical aberration induced by the flat optical surface of the transparent workpiece. This is important by writing waveguides, nanostructuring, selective laser etching (SLE), optical data storage implemented with focusing up to 0.8NA, when spherical aberration and the subsequent reduction in energy concentration, as well as lowering physical lateral and longitudinal resolution, lead to the degraded performance. A solution to compensate for spherical aberration by deep high NA focusing, for example up to 4 mm in glass with 0.8NA by writing waveguides, is suggested in the form of an aplanatic objective of the patented optical design aplanoXX, supplied with a protective window. The function of spherical aberration of this objective matches the aberration function of the flat optical surface of the transparent medium; therefore, exact compensation of spherical aberration is realized simultaneously with focusing in the medium at a given depth for an arbitrary NA with providing a single spherical wavefront inside the medium.

Due to flexibility of the adjustable optical system, the aplanoXX objective can be adapted to operate with fused silica, glasses, sapphire, silicon carbide, silicon, media of eye in applications based on ultra-short pulse lasers of 1030 nm, 800 nm, 515 nm. The replaceable window protects the optics from damages by particles ejected during material processing.

The poster presents an analysis of high NA focusing inside a transparent medium at different depths on the examples of fused silica, as well as the experimental results for the writing waveguides, confirming effectiveness of the method of high NA focusing with spherical wavefront inside the transparent medium.